of work to do, this timestamp could get behind times, and send the
thread to sleep, despite being behind on work.

Fix this, by updating the timestamp whenever we run out of work.

Add a parameter ("expiry_sleep") to control how long time the thread
will sleep so this can be tuned down on high-load servers.

Inspired by:	sky



git-svn-id: http://www.varnish-cache.org/svn/trunk/varnish-cache@5583 d4fa192b-c00b-0410-8231-f00ffab90ce4